The relation between occupational physical activity, myocardial infarction and all-cause mortality: dependency of leisure time physical activity

Introduction
Occupational physical activity (OPA) is observed to be positively associated with cardiovascular disease and mortality. However, it is unsettled if leisure time physical activity (LTPA) modifies this association

Methods
In The Copenhagen City Heart Study, a cohort of 8496 males and females, age 25-66 years without known cardiovascular disease in 1976-78 and 1981-83, the association between OPA and fatal or non-fatal myocardial infarction (MI) as well as all-cause mortality was investigated by sex and LTPA stratified multivariate adjusted Cox proportional hazards ratios (HR). OPA at both examinations was combined, and persons categorized into low, moderate and high levels

Results
During a median follow-up of 18.4 years for all-cause mortality and 17.8 years for MI, 787 suffered a MI and 2888 died. Among males, moderate and high OPA increased the risk of MI and all-cause mortality. High OPA increased the risk of all-cause mortality among males with low (HR=1.51; 95% confidence interval (95%CI) 1.08-2.10) and moderate (HR=1.30; 95%CI 1.04-1.61) LTPA, but not among males with high LTPA (HR=0.99; 95%CI 0.78-1.26). Similar tendencies were found for MI. The effect of OPA differed by gender for both MI and all-cause mortality (interactions, p<0.02) with no significant associations found among females

Discussion
OPA was associated with increased risk in males with low and moderate LTPA, but not in males with high LTPA. Results suggest that high LTPA perhaps through increased cardiovascular fitness yields protection against harmful effects of high OPA. The different relationship between OPA and MI and mortality between genders may be due to dissimilar occupational physical exposures